The present invention relates generally to switching power supply circuits. In particular, the invention relates to circuits that supply power to loads in continuous conduction mode and discontinuous conduction mode.
Portable electronic devices typically require the application of a regulated DC voltage in a predetermined range of voltages for satisfactory operation. Many electronic devices rely on unregulated DC supplies such as lithium-ion batteries as a power source. Batteries generally provide a voltage that is substantially fixed over short time periods but slowly decreases throughout its useful lifetime. Consequently, battery voltage is often transformed to a regulated supply voltage having a different voltage value to ensure proper operation of the electronic device.
The prior art teaches many ways to accomplish this conversion. For example, some portable electronic devices use arrays of capacitors (e.g., charge pumps) to convert the source voltage into a voltage with a different polarity or magnitude. Other devices use switching power supplies to provide a regulated voltage for proper operation. Switching losses inherent in such supplies can limit the power efficiency.
Certain portable electronic devices utilize unregulated DC supplies that are sensitive to current backflow. For example, lithium-ion batteries can experience heating problems or can be damaged if current flows back into the battery. Therefore, it is desirable to provide a switching power supply that prevents current backflow and minimizes switching losses.
The present invention relates to a circuit and method for powering DC devices using DC voltage sources. The present invention provides an improved switching power supply that has reduced switching losses and prevents load current reversal under light load conditions. The circuit operates using constant ripple current regulation in continuous and discontinuous mode operation. In discontinuous mode, this is accomplished using pulse-frequency mode modulation. A rectifier circuit prevents current backflow into the DC voltage source, which otherwise can cause overheating and device failure.
In one aspect, the invention relates to a circuit for generating a regulated output voltage. In one embodiment, the circuit includes an inductor, a first switch, a pulse generator and a rectifier circuit. The inductor has a first terminal and a second terminal. The first switch has a first terminal to receive a first reference voltage, a second terminal in communication with the first terminal of the inductor, and a control terminal for receiving a first control signal. The pulse generator has an input terminal and an output terminal, which provides the first control signal, in communication with the control terminal of the first switch. The rectifier circuit has a first control input terminal in communication with the output terminal of the pulse generator, a second control input terminal in communication with the second terminal of the second switch and a third control input terminal to receive the second reference voltage.
In one embodiment, the rectifier includes a first comparator, a logic module, and a second switch. The first comparator has a first terminal in communication with the second control input terminal, a second terminal in communication with the third control input terminal, and an output terminal. The logic module has a first input terminal in communication with the first control input terminal, a second input terminal in communication with the output terminal of the first comparator, and an output terminal to provide a second control signal. The second switch has a first terminal to receive a second reference voltage, a second terminal in communication with the first terminal of the inductor, and a control terminal to receive the second control signal. In another embodiment, the second terminal of the first comparator receives a small negative voltage.
In another embodiment, the pulse generator includes an adaptive pulse generator and an OR gate. The adaptive pulse generator has an input terminal in communication with the input terminal of the pulse generator and an output terminal. The OR gate has a first input in communication with the output terminal of the adaptive pulse generator, a second input in communication with the input terminal of the pulse generator, and an output terminal in communication with the output terminal of the pulse generator. In yet another embodiment, the pulse generator also includes a comparator. The comparator has a first terminal in communication with the second terminal of the inductor, a second terminal to receive a third reference voltage, and an output terminal connected with the input terminal of the adaptive pulse generator. In still another embodiment, the pulse generator includes an overcurrent detector having an input terminal connected to the first inductor terminal and an output terminal connected to the first switch control terminal.
In still another embodiment, the logic module includes a flip-flop and a NOR gate. The flip-flop has an input terminal in communication with the output terminal of the first comparator, a reset terminal in communication with the first control input terminal, a data terminal to receive the first reference voltage, and an output terminal. The NOR gate has a first NOR input terminal in communication with the first control input terminal, a second NOR input terminal in communication with the output terminal of the flip-flop, and an output terminal in communication with the control output terminal.
In another embodiment, the second terminal of the second comparator is in communication with the second terminal of the inductor through a voltage divider network. In one embodiment, the voltage divider network includes a first resistor and a second resistor. The first resistor has a first terminal coupled to the second terminal of the second inductor, and a second terminal. The second resistor has a first terminal coupled to the second terminal of the first resistor and a second terminal to receive the second reference voltage.
In another aspect, the present invention relates to a method for generating a regulated output voltage. The method includes the step of applying a first reference voltage to a series combination of an inductor and a load if an elapsed time is less than a predetermined time or if a voltage across a load is less than a first predetermined voltage. A second reference voltage is applied to the series combination of the inductor and the load if the elapsed time is greater than the predetermined time and if the load voltage is not less than the first predetermined voltage. The application of the second reference voltage is terminated if the voltage across the series combination of the inductor and the load exceeds a second predetermined voltage or if the voltage across the load does not exceed the first predetermined voltage.